Aluminum is typically produced by the Hall-Heroult electrolytic reduction process wherein aluminum oxide dissolved in molten cryolite is electrolyzed at a temperature of from 900.degree. C. to 1000.degree. C. The process is conducted in a pot-type reduction cell which typically comprises a steel shell the interior of which is provided with an insulating lining of a suitable refractory material, which is in turn provided with a lining of carbon, the latter being in contact with the molten constituents. One or more anodes typically made of carbon, connected to the positive pole of a source of direct current are suspended within the cell and one or more iron conductor bars connected to the negative pole of a source of direct current are typically embedded in the carbon lining comprising the floor of the cell causing the carbon lining to become cathodic upon application of current. Molten aluminum is continuously electrolyzed out of the aluminum oxide-cryolite melt and collects on the cathodic carbon floor of the cell and is continuously or periodically withdrawn. A shallow pool or pad of molten aluminum is always maintained on the carbon floor of the cell which molten aluminum pad, since it is in electrical contact with the carbon floor, functions as the active cathodic surface.
Satisfactory electrical conductance between the carbon lining and the molten aluminum pad is hindered by carbon surface effects and by the accumulation of undissolved bath material on the carbon floor of the cell which sludge or muck forms an insulating layer increasing the voltage drop across the cell and lowering its power efficiency.
In order to enhance current conductance from the cathode supply bus to the molten metal pad, electrode elements formed, inter alia, of electrically conductive refractory hard metal have been proposed and are described, for example, in U.S. Pat. No. 3,156,639. It has also been proposed to bond a thin layer of electrically conductive refractory hard metal to the carbon lining as described for example, in U.S. Pat. No. 3,856,650. Furthermore, it is known to line the cell by cementing electrically conductive refractory hard metal tiles to the carbon lining.
However, bonding a layer of electrically conductive refractory hard metal or cementing refractory hard metal tiles to the carbon lining is disadvantageous in that this would not prevent impairment of current conductance to the molten metal pad caused by sludge accumulation and more importantly the refractory hard metal when bonded or cemented to the carbon lining will tend to fracture due to the difference in coefficients of thermal expansion between the refractory hard metal and the carbon.